The goal of this project is to determine the mechanistic differences in MEK/ERK signaling that mediates growth arrest versus cell proliferation in response to aberrant Ras and/or Raf signals. Aberrant activation of the Ras/Raf/MEK/ERK pathway is a central feature in many cancers. However, paradoxically, sustained activation of the pathway induces cell cycle arrest and senescence in normal cells and certain cancer types. It has been proposed that growth arrest acts as a defense mechanism against Ras/Raf-mediated carcinogenesis and that overcoming this growth arrest barrier is a necessary step in tumor progression. Our understanding of this key event in carcinogenesis, controlled at the level of MEK/ERK, is currently quite limited. We have begun to address the underlying mechanisms of MEK/ERK signaling using normal and Ras/Raf-responsive tumor cells as models. The first intriguing finding is that ERK can mediate growth arrest by utilizing not only its canonical kinase activity but also, as yet unidentified, non-catalytic functions. We recently reported that oncogenic Raf signal-induced growth arrest is abrogated by ERK1/2 depletion but introduction of catalytically disabled mutant ERK into ERK1/2- depleted cells can selectively restore the growth arrest phenomenon. Based upon a hypothesis that MEK/ERK would interact with specific proteins to mediate growth arrest signaling, we conducted tandem affinity purification and identified mortalin as a potential negative regulator of MEK/ERK-growth arrest signaling. Our preliminary studies show that mortalin binds to inactive, but not active, MEK and that mortalin depletion increases basal as well as Raf-induced MEK/ERK activity. In addition, mortalin depletion promotes growth inhibitory signaling whereas mortalin overexpression exerts the opposite effects. These preliminary studies suggest that MEK/ERK utilizes a unique signaling mechanism to mediate growth arrest, for which mortalin has a negative-regulatory role possibly via its physical interaction with the pathway and ERK modulates a mechanism that requires its non-catalytic function. To test these hypotheses, we propose to (i) determine the role of mortalin in the regulation of Raf/MEK/ERK-mediated growth arrest signaling by gain or loss of function studies in normal and K-Rasor B-Raf mutated tumor cells and by comparing mortalin expression levels with altered MEK/ERK activity in tumor tissue specimens; (ii) determine whether mortalin regulates the Raf/MEK/ERK pathway by differentially sequestering MEK1 or MEK2; and (iii) determine molecular mechanisms underlying the non-catalytic ERK functions. This study will enhance our knowledge of the specific mechanisms of MEK/ERK signaling that interrupts Ras/Raf-driven carcinogenesis.